feat(tui): background running compose with Command-B

[danielkov]

Summary

• add Command+B to detach the newest running foreground compose call
• route detachment out of band through ACP so it works while the session actor is driving the turn
• preserve turn cancellation before detachment and independent cancellation afterward
• keep detached calls cancellable with the existing Ctrl+K and close paths
• document the shortcut and bump Kit to 0.1.86

Dependency

• uses released agentkit-task-manager 0.10.6 from crates.io
• AgentKit implementation merged in feat(task-manager): detach running foreground tasks danielkov/agentkit#10 and released by chore(release): task manager 0.10.6 danielkov/agentkit#11

Tests

• cargo test --locked --quiet
• cargo fmt --all -- --check
• cargo clippy --locked --lib --tests -- -D warnings
• git diff --check